Light sensing techniques are used in many applications. Some applications, such as turning on a street light after sunset, do not require fast light sensing. In other applications, such as turning on a backlight of a liquid-crystal display (“LCD”) of a portable device such as a mobile phone, personal digital assistant (“PDA”) or camera, if the ambient is not bright enough to view the LCD, a fast response time is desirable.
For example, photodetectors are used to sense whether a flash needs to be activated while taking a picture with a camera. If the scene is too dim, the photodetector senses a low light condition and sends a signal that activates the camera's flash. However, the flash generates intense light that can saturate the photodetector. Photodetector circuits often have slow recovery, and the photodetector might provide an inaccurate reading if another picture is taken before the photodetector has relaxed. In other words, a first flash can cause the flash to be kept off when camera takes a second picture, even if the lighting of the second picture is dim and light from the flash would be desirable.
Infrared Light Data Sensing (IrDA) applications also use photodetectors. Infrared (“IR”) data pulses are transmitted at various frequencies, depending on the IrDA standard being used. For example, data is transferred at 115 Kbps in slow IR (“SIR”) mode, at 1 Mbps in medium IR (“MIR”) mode, and at 4 Mbps in fast IR (“FIR”) mode. Photodiodes with faster response times are desirable for use at the faster data transfer rates.
One type of photodetector that is used in conjunction with a backlight of an LCD is a PIN diode. “PIN” stands for p+-type, intrinsic, n+-type. Switching speed is especially critical for fast data rate applications. It is desirable to minimize the capacitance at the sensing node of photodiode. However due to the nature of the parasitic PIN capacitance that varies with reverse voltage, it is a difficult to control the switching speed, especially in a situation when reverse voltage of the photo device is less than 1V. At such low voltages, the diode capacitance can greatly increase, resulting in a slower fall time and slowing switching speed. A slow fall time limits the data rate received by the PIN diode.
A PIN diode used as a photodiode in light sensing application is usually reversed biased. When reverse bias is applied, a dark current of about 1 nano-Amp to about 3 nano-Amps flows if the photodiode is not illuminated. Illuminating the diode with light of appropriate wavelength(s) increases the reverse current though the diode by generating charge carriers that are swept through the reverse junction. In the illuminated condition, the reverse current increases linearly with increasing reverse voltage (before avalanche breakdown occurs).
FIG. 1A is a circuit diagram of a prior art light sensing circuit 100. When light, represented by an arrow 102, illuminates a PIN diode 104, a photocurrent is generated. The photocurrent flows through a resistor 105 to produce a voltage equivalent VA at an input to a gain stage 106. The voltage equivalent VA is amplified by amplifier 106 and passed through a low-pass filter (“LPF”) 108. The LPF, 108 averages out the photodiode response, which can have relatively fast-varying components due to light noise and flicker, for example, so that only the relatively time invariant (i.e. essentially DC) component of the voltage equivalent is used. The filtered photodetector output 109 is coupled to a first input 110 of a hysteresis comparator 112. A voltage reference Vth (“threshold voltage”) is coupled to a second input 113 of the hysteresis comparator 112. The hysteresis comparator 112 basically provides an output signal 114 in a first state (e.g. VCC) if the filtered photodetector output 109 is greater than the threshold voltage Vth, and provides the output signal 114 in a second state (e.g. ground or −VCC) if the filtered photodetector output 109 is less than the threshold voltage Vth, or vice versa. The hysteresis in the hysteresis comparator 112 keeps the backlight from flickering on and off if the filtered photodetector output 109 jitters across the threshold voltage Vth.
The output 114 of the hysteresis comparator 112 is coupled to an output buffer, also known as a driver or driver stage, 116, and then to an output 118. If the output signal is in the first state, the backlight to the LCD is turned off, and if the output signal is in the second state, the backlight to the LCD is turned on, or vice versa. However, the switching speed of the PIN diode 104 depends on the reverse bias capacitance of the PIN diode, which is a function of the reverse bias voltage.
FIG. 1B is a plot of capacitance versus reverse bias voltage for an exemplary PIN diode. The capacitance, which was measured at a frequency of 1 MHz, is on a linear scale, and the voltage is on a logarithmic scale. The capacitance of the PIN diode equals the permittivity of the PIN junction multiplied by the area of the junction, divided by the distance between the “plates” (conductive regions) of the PIN diode. Thus, as the reverse bias voltage increases, the distance between the conductive regions of the PIN diode increases, reducing the capacitance. Therefore, it is desirable to provide photodetector circuits with improved switching (response) speeds.